This invention relates to a spindle control system for machine tools having a variable speed gear mechanism.
FIG. 1 shows an embodiment of a prior art spindle control system used for a spindle with a variable speed gear mechanism.
Position command .theta..sub.S * or speed command .omega..sub.S * for a spindle 9 are inputted to a speed command circuit 1. The speed command circuit 1 detects a rotating position .theta..sub.S of the spindle 9 through a spindle rotary encoder 8 and calculates a speed command .omega..sub.S *=Kp (.theta..sub.S -.theta..sub.S *) by means of a position loop gain coefficient Kp when the command is the position commands .theta..sub.S *. The speed command .omega..sub.S * is converted into a speed command .omega..sub.M *=G.sub.M /G.sub.S .times..omega..sub.S * for a motor shaft by using the gear ratio G.sub.M /G.sub.S between the spindle 9 and the motor shaft of the gears presently engaged, and then is inputted to a speed control circuit 2. The speed control circuit 2 detects the position .theta..sub.M of the motor shaft by means of a motor shaft rotary encoder 4, and controls an electric current passing through a motor 5 by a power converter 3 so that the timewise change .omega..sub.M of the position .omega..sub.M becomes conincided with .omega..sub.M *.
When the gear ratio G.sub.M /G.sub.S is to be changed in a conventional spindle control system as described above, the motor 5 should be stopped temporarily and gear 7 which is slidable an the axial direction is moved by an actuator 6 so as to disengage the gear 7, and the speed command .omega..sub.S * is given to swing the motor shaft at a low speed while moving gears to be engaged as newly instructed.
As described above, the conventional spindle control system could not cause th spindle to agree with the motor shaft in position and in speed as instructed by a command from outside. Therefore, when the gears were to be newly engaged, both the motor shaft and the spindle had to be inconveniently suspended temporarily in order to make the peripheral velocities of the gears which are to be engaged agree with each other. However, when the gears were to be engaged in the prior art, both gears were simply pushed to press-fit each other without aligning angles thereof to thereby apply unnecessary loads on the teeth of the gears as well as to require much time for the gear ratio changing.
There has been proposed a system which allows gears to be engaged at a new gear ratio if the difference of the speed of both gears in rotation remains within a certain scope by providing a mechanical synchro (motor) unit on the system. For example, Japanese Patent Application No. 168637/1980 proposed a circuit which can detect agreement of the revolution number of the gears for such purpose. However, such systems are defective in that the cost of gears becomes high and it requires a larger space for installation.